1. Field of the Invention
The present invention relates to a semiconductor device having a circuit composed of a thin film transistor (hereinafter referred to as a TFT), and to a method for manufacturing the same. For example, the invention relates to an electronic device which mounts a light emitting display device having a TFT and an organic light emitting element as its component.
In this specification, the term “semiconductor device” refers to a device in general that utilizes semiconductor characteristics to function, and electro-optical devices, semiconductor circuits, and electronic devices are all included in the semiconductor device.
2. Related Art
In these years, research on a light emitting device having an EL element as a self-luminous light emitting element is activated. The light emitting device is also referred to as an organic EL display or an organic light emitting diode. Since these light emitting devices have characteristics such as high response speed that is suitable for movie display, low voltage, and low power consumption driving, they attracts an attention for a next generation display including new generation's cellular phone and personal digital assistant (PDA).
An EL element using a layer containing an organic compound as a light emitting layer has a structure in which a layer containing an organic compound (hereinafter referred to as an EL layer) is interposed between an anode and a cathode. Electro luminescence is generated in the EL layer by applying an electric field to the anode and the cathode. Luminescence obtained from the EL element includes luminescence generated in returning to a ground state from a singlet excited state (fluorescence) and luminescence generated in returning to a ground state from a triplet excited state (phosphorescence).
However, sufficient luminance is not obtained in a conventional light emitting element using a layer containing an organic compound as a light emitting layer.
Light generated in an EL layer is extracted, regarding an anode side or a cathode side as a display surface. On this occasion, light is partly reflected at an interface among different material layers while passing through various material layers and a substrate. As a result, there is a problem that initial emission of light to be transmitted to outside of an element is reduced by several tens percent and luminance is kept low.
Thus, Reference 1 and Reference 2 by the present applicant propose an element structure for improving light extraction efficiency (Reference 1: Japanese Patent Laid-Open No. 2002-352950, and Reference 2: Japanese Patent Laid-Open No. 2002-229482).
Since luminous efficiency is low in the conventional light emitting element using a layer containing an organic compound as a light emitting element, the amount of current for obtaining desired luminance increases and power consumption also increases. Increase in power consumption greatly affects an element life, typically, shortens a half-life time of luminance. Therefore, the conventional light emitting element using a layer containing an organic compound has a problem that needs to be solved regarding element stability.